The present invention relates to photolithographic imaging. More particularly, the invention relates to apparatus and methods for measuring various optical and kinetic variables in a photoresist masking process to characterize the process and to control the operation of the equipment used in the process.
Photoresist masking is employed in a variety of photolithography or optical lithography processes. Generally, a photolithography exposure tool projects an image into a photosensitive layer placed on a substrate. For example, an integrated circuit is built from numerous layers formed on a substrate, many such layers defined through photolithographic imaging. In addition to integrated circuit manufacturing, photoresist masking is used in other manufacturing applications such as for forming flat panel displays, thin film magnetic disk heads, optoelectrical devices, micro-manufacturing and advanced packaging.
The pattern that is projected into the photosensitive layer is contained on a mask that is placed within the photolithography exposure tool. The mask may, as in integrated circuit manufacturing, for example, be made from patterned chrome placed on glass. The pattern is transferred onto the substrate by projecting an image of the mask onto a photosensitive layer placed on the substrate. For integrated circuits, the photosensitive layer is generally a photoresist layer placed on a semiconductor wafer. Typically photoresist comprises nonvolatile materials, including a polymer resin and a photo active agent, dissolved or otherwise dispersed within a volatile solvent.
Many different types of tools can be used to project an image of the mask onto a substrate. U.S. Pat. Nos. 3,951,546 and 4,068,947 describe a tool which scans a slit of light across the mask, through an annular unit magnification optical projection system onto a wafer. The disclosure of both these patents is expressly incorporated herein by reference. This tool, commonly known as a scanner, generally uses a broadband light source with a selectable wavelength range and a fixed degree of coherence and has a catoptric or a catadioptric optical system with a fixed numerical aperture.
Another type of photolithography tool exposes a small area of a wafer at one time. The wafer is then stepped to a new location and the exposure is repeated. This type of tool is called a step-and-repeat projection system, or stepper. U.S. Pat. No. 4,425,037, the disclosure of which is expressly incorporated herein by reference, describes a unit magnification stepper with a broadband light source of fixed wavelength range and coherence and fixed numerical aperture. Other steppers are reduction projection systems in which the mask size is larger by a fixed factor than the printed image. These systems generally employ a fixed monochromatic light source, such as that disclosed in U.S. Pat. No. 4,206,494, and a sophisticated objective lens such as that disclosed in U.S. Pat. No. 4,616,908, the disclosures of each of which are expressly incorporated herein by reference. All of the above-mentioned types of photolithography exposure tools are used to selectively expose a photoresist coated substrate, a wafer, generally placed on substrate holder, such as a vacuum chuck, within the tool.
The photoresist itself is an important part of the overall optical process and the absorption of light by the photoresist will impact the amount of energy transferred from the photolithography tool to the photoresist. Furthermore, the reflectivity of the materials underlying the photoresist will effect the amount of energy transferred to the photoresist. Thus, it would be desirable for a photolithography exposure tool to adjust illumination in response to the amount of absorption of light by the photoresist and the reflectivity of the underlying substrate.
The absorption of energy by photoresist is dependent on a number of factors. Photoresist chemical components and structures vary between different commercial photoresist suppliers, thus effecting the amount of absorption. Also, batch to batch variation exists even from one supplier. Further variation can be added by photoresist pretreatment, such as a prebake used to dry the resist, prior to exposure. Prebake variables such as time and temperature can ultimately effect the absorption of light by photoresist. Photoresist thickness, the photoresist age and the time between photoresist coating and exposure will also add variation. Furthermore, absorption of energy by photoresist decreases as the photoresist is exposed to light, therefore adding an exposure dependent factor.
Thus the absorption of energy by photoresist is a complicated system dependent on many factors. In order to characterize photoresist absorption, various characterization coefficients and constants are known to those skilled in the art. Typically such characterization parameters are calculated by measuring the transmission of light through a photoresist coated glass substrate. As an example, for positive photoresist, absorption has been characterized by calculating photoresist parameters such as exposure dependent or bleachable resist absorption coefficient A, exposure independent or non-bleachable resist absorption coefficient B, and optical sensitivity or resist kinetic exposure rate constant C. Equations for such parameters are known in the art and shown in Equations 1-3, EQU A=(1/d)ln[T(.infin.)/T(0)] Eq. 1 EQU B=-(1/d)lnT(.infin.) Eq. 2 ##EQU1## where d is the photoresist thickness, T(0) is the internal transmittance of the photoresist before exposure, T(.infin.) is the internal transmittance of completely exposed photoresist, and I.sub.o is the illuminating source intensity. Other characterization equations may also be used. For example with polysilane photoresist, non-linear bleaching properties have been reported as shown in Equation 4. EQU A=[0.5+1.4(M.sub.c -0.4)]A.sub.c Eq. 4.
where A is the bleachable absorption coefficient M.sub.c is the concentration of unbleached polysilane with an absorbance of A.sub.c.
The reflectivity of the material underlying the photoresist is another variable that impacts the amount of energy delivered to the photoresist. A more reflective substrate under the photoresist will result in higher energy levels being delivered to the photoresist. For example, during integrated circuit manufacturing a variety of films may be stacked on the substrate. The reflectivity of the combination of the substrate and stacked films can vary between manufacturing steps because different films are on the substrate during different steps. The reflectivity may even vary at the same manufacturing step because the film thickness or properties may vary from substrate to substrate or batch to batch.
Reflectivity measurements are generally used in photolithography for matching the photoresist thickness to the reflectivity and for measuring film thickness. Currently, however, reflectivity is not measured by the projection tool, but rather by separate off-line specialized instruments that are commercially available such as the Nanometrix Nanospec, Prometrix FT 500, and Tencor TF. Because reflectivity affects the ability of a photolithography exposure tool to deliver a constant energy into the photoresist, it is desirable to more directly utilize this information to characterize photoresist and to control the exposure energy delivered by a photolithography exposure tool.
The goal of each photolithography tool is to deliver precisely controlled amounts of energy into the photoresist. The most common lithographic result which must be controlled is the linewidth of critical dimensions that are printed on a substrate. Typically the amount of energy delivered to the photoresist, and thus the critical dimension printed, is adjusted by setting the photolithography exposure tool to a fixed exposure time. A constant lithographic result, however, is difficult to achieve in the face of changes in the properties of the lithographic tool, the photoresist and the substrate.
Characterization parameters and methods are generally used for photoresist modeling and research. Currently, these parameters are not measured by the photolithography exposure tool, but rather by separate off-line specialized instruments, one such instrument is the PACScan is commercially available from Site Services. It would be desirable to measure characterization parameters in situ using the photolithography exposure tool because these parameters affect the ability of a photolithography exposure tool to deliver a constant energy into the photoresist.
The in-situ measurement apparatus and methods of the present invention provide more accurate photoresist and reflectivity measurements compared to prior methods because in accordance with the present invention, the present inventor has recognized that accuracy is increased when the measurements are made with the photolithography exposure tool that is used to expose the lithographic pattern rather than with a separate off-line instrument. In accordance with the present invention, the present inventor has also recognized that by improving the accuracy of these measurements, the resulting data may be used to control the exposure of the photolithography tool. The resist parameters and reflectivity measurements obtained from separate off-line specialized tools have several problems. First, the light sources used for the resist and reflectivity measurements are not calibrated to the light source used in the exposure tool. Also, the calibration would have to be constantly updated and would suffer from inaccuracies because the optical properties of the light sources change over time. For example, light intensity changes between sources and with time. Also, in broadband sources the spectral output, i.e. intensity vs. wavelength, changes between sources and with time. The reflectivity measurement would pose additional problems because a measured reflectivity value is a function of the numerical aperture of the illumination source. Thus, unless the numerical aperture of the measurement device is the same as that of the photolithography exposure tool, and it usually is not, the measurement will suffer from inaccuracies. Furthermore, detectors on different tools may give different measurements, because the measurement accuracy of detectors changes as the light source wavelength output varies, especially for broadband sources. The in situ measurement apparatus and methods of the present invention eliminate these problems.
Generally existing tools emphasize controlling the energy directed toward the substrate by using a set light intensity and varying the exposure time. An exposure time is typically set based on prior experience of the user or by measuring the resulting critical dimensions on a test substrate to determine the exposure for production substrate. More accurate lithographic results, though, would result from controlling exposure time in response to the actual energy absorbed by the photoresist, thus compensating for changing properties in the lithographic process. Present photolithography exposure tools fail to adequately compensate for the changing lithographic properties and thus linewidth accuracy is lost.
A closed loop in situ method for exposing photoresist is disclosed in "Use of Diffracted Light From Latent Images to Improve Lithography Control," SPIE Vo. 1464 Integrated Circuit Metrology, Inspection, and Process Control V, p. 245-57(1991). This method utilizes the latent image of a diffraction grating that is projected into photoresist on a wafer. Laser light is directed towards the grating during exposure. The intensity of the light diffracted from the latent image is measured during exposure. The diffracted intensity varies with the exposure because as more energy is delivered to the photoresist, the refractive index of the photoresist changes. When the measured diffracted intensity reaches a predetermined limit, exposure is halted. Thus, exposure control becomes dependent on an in situ measurement.
This latent image method has several disadvantages. First, the absorption of light by the photoresist is not directly measured, but rather is only indirectly measured by the change in refractive index. The latent image method does not adequately account for bleachable absorption coefficients and non-bleachable absorption coefficients. It is desirable to directly measure absorption so that the photoresist can be fully characterized such as, for example, by using the characterization equations presented above and so that exposure control may be more directly related to the absorption of energy by the photoresist. The latent image method also requires a diffraction grating pattern to be added to lithographic process. In addition, another light source, the laser, is required, thus adding components and potential calibration inaccuracies. Finally, it is desirable to provide a more direct method to compensate for substrate reflectivity variations.